The invention relates to the provision of a hermetic seal for the strain gages of a load cell.
A conventional load cell of the bending beam type includes a live end block, a dead end block, and plural bending beam elements extending between and interconnecting the live end block and the dead end block. Foil strain gages or the like are affixed, e.g. adhesively mounted, on the thin sensing sections of the bending beam elements. The strain gages are connected by wires to the sensing electronics, e.g. a weighing bridge circuit. The dead end block is bolted to a stationary support, while a live load is introduced into the live end block. The live load, i.e. the load to be weighed, causes the bending beam arrangement to deflect, thereby inducing corresponding strains in the bending beam elements. The strains of the bending beam elements are sensed and measured by the strain gages, whereby the weight of the applied live load can ultimately be determined.
Such bending beam load cells are often used in so-called single point applications, in which a single load cell is provided to weigh an entire load. For example, the entire load of a weighing platform is introduced into the live load introduction end of the bending beam load cell, in order to weigh the live load on the weighing platform.
In most applications, it is necessary to seal and protect the delicate strain gages from environmental influences which include mechanical abrasion or other mechanical damage, corrosion and etching by harsh chemicals and the like, oxidation, and other undesirable influences that destroy or damage the strain gages or interfere with the proper functioning thereof. It has become known to apply a polymeric seal layer, such as a layer or film of polybutylene over the strain gages. It has further become known to seal, embed, or pot the strain gages in a sealing mass of room temperature vulcanizing (RTV) silicone or the like.
While such known sealing methods provide an effective environmental seal, such measures are temporary and not very robust. Namely, such polybutylene, silicone, or other polymeric seal layers have been found to leak, breakdown, or peel off under harsh environment applications, such as in the food production and preparation industry, and in the chemical processing and handling industry. In such harsh environment applications, the load cells, and particularly the seal areas provided over the strain gages, are exposed to harsh or severe chemicals, as well as frequent cleaning, for example using solvents and the like, and using mechanical scrubbing or pressure washing procedures. Under such severe conditions, conventional polymeric seals on the strain gages of load cells have been found to be unsatisfactory due to a short reliable operating lifespan before leakage or peeling of the seal material occurs.
To provide a longer-term, more-durable hermetic seal, in comparison to the above mentioned polymeric seals, it has also become conventionally known to encapsulate or seal the strain gages with a stainless steel seal member. A problem that arises when trying to use a metal seal member, is that the seal member itself takes up some of the stress and thus influences the development of the strain in the sensing sections of the bending beam arrangement. The particular conventional load cell configuration that has become known for addressing the above problems is schematically illustrated in a simplified manner in present FIG. 1.
As shown in FIG. 1, a conventional bending beam load cell 1xe2x80x2 having a stainless steel hermetic seal is a rather complicated triple beam arrangement 2xe2x80x2 including an upper beam element 5xe2x80x2, a lower beam element 6xe2x80x2, and a central beam element 7xe2x80x2, respectively extending and connected between the live load introduction end 3xe2x80x2 and the dead end 4xe2x80x2. The entire bending beam arrangement 2xe2x80x2 is machined from a single monolithic block of stainless steel. The complicated configuration as shown in FIG. 1 results in a rather high machining effort and cost. Particularly, the upper and lower beam elements 5xe2x80x2 and 6xe2x80x2 are substantially straight beam elements, while the central beam element 7xe2x80x2 is a circular ring element. The upper and lower beam elements 5xe2x80x2 and 6xe2x80x2 maintain a parallelogram configuration, and passively handle off-center load application moments. On the other hand, the central beam element 7xe2x80x2 including the circular ring element is the active bending element that takes up the load to be measured.
For measuring the strain of the circular ring element and thereby measuring the applied live load, strain gages 9xe2x80x2 are applied on the inner circumferential surface of the ring element of the central bending beam element 7xe2x80x2. In FIG. 1, the strain gages 9xe2x80x2 are merely schematically indicated as a dashed line. Actually, the strain gages 9xe2x80x2 are not visible from the outside, because they are encapsulated and hermetically sealed by a cylindrical sleeve or tube 10xe2x80x2 of stainless steel that is arranged in the interior of the ring element of the central bending beam element 7xe2x80x2. The hermetic seal tube or sleeve 10xe2x80x2 is welded around the edges to the ring-shaped element of the central bending beam element 7xe2x80x2, to achieve the hermetic seal with a complete stainless steel enclosure. Thus, only a stainless steel surface is exposed to the environment, and the strain gages are hermetically sealed therein. An electrical cable 12xe2x80x2 is connected and sealed into the dead end 4xe2x80x2 to conduct the weighing signals provided by the strain gages 9xe2x80x2 and pre-processed by electronic circuitry in the load cell 1xe2x80x2.
While the conventional hermetically sealed stainless steel bending beam load cell schematically illustrated in FIG. 1 provides an effective durable hermetic seal and is suitable for use in harsh or extreme environmental conditions, it also suffers several disadvantages. The machining required for the complex configuration of the load cell results in rather high machining efforts and cost. Also, the complex configuration with several interior surfaces, corners, notches, grooves, and the like forms spaces in which liquids will puddle. This is a disadvantage in the food processing and chemical processing industries, in which the load cells are frequently exposed to various liquids during use and during cleaning procedures. The puddling and accumulation in the xe2x80x9cnooks and cranniesxe2x80x9d of the complex configuration of the load cell make it difficult to keep the load cell clean, and make higher demands on the long term corrosion resistance and hermetic sealing.
Also, the complex triple beam configuration including a ring-shaped strainable element necessarily leads to a rather large profile height for a given beam length and load capacity. Moreover, this triple beam arrangement, and the use of a cylindrical internal stainless steel sleeve or tube to provide the hermetic seal, make it difficult to achieve a low load capacity. Namely, due to the influence of the hermetic seal tube or sleeve 10xe2x80x2 and due to the arrangement of three bending beam elements 5xe2x80x2, 6xe2x80x2 and 7xe2x80x2, a certain minimum load is required to sufficiently strain the central bending beam 7xe2x80x2 for an accurate weighing result. The minimum capacity for such load cells is typically about 20 kg, although claims of a capacity down to about 6 kg have been noted.
In view of the above, it is an object of the invention to provide a load cell with a hermetic seal for the strain gages thereof, while achieving a lower capacity range, a simpler configuration, a reduced machining effort, a reduced cost, a lower profile height, and an easy retrofit capability to replace previously existing load cells. The invention further aims to provide a hermetic seal configuration for a load cell, that achieves improved separation or isolation of the strain from the seal elements, so as to minimize the influence of the hermetic seal elements on the strain development in the sensing sections of the load cell. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification. The attainment of these objects and advantages is, however, not a requirement of the present invention.
The above objects have been achieved according to the invention in a load cell comprising a load cell body including a strainable sensing section, a strain gage arranged on a surface of the sensing section of the load cell body, and a cup-shaped seal cap that is arranged on the surface of the load cell body so as to enclose the strain gage. The seal cap is heat-fused onto the surface, so as to form a hermetically sealed joint therebetween. The relevant surface of the load cell body according to one feature of the invention is a flat planar surface. According to another feature of the invention, this surface of the load cell body on which the strain gage and the cup-shaped seal cap are arranged is an outer surface that faces outwardly away from a median plane extending longitudinally through the load cell body. The heat-fusing of the seal cap onto this surface of the load cell body is preferably achieved by laser welding.
According to a further detailed embodiment of the invention, the above objects have been achieved in a load cell comprising a live load introduction end, a dead end, at least one bending beam element extending and connected between the live end and the dead end, at least one strain gage arranged on a surface of the bending beam element at a respective sensing section thereof, and a cup-shaped seal cap that is arranged on the beam surface so as to enclose the respective strain gage therein, and that is heat-fused (e.g. laser welded) onto the beam surface so as to form a hermetically sealed joint therebetween.
With this structure and arrangement of the load cell, the entire load cell is hermetically sealed, and particularly the strain gages are enclosed and hermetically sealed by the seal cap or caps. The structure and the fabrication steps for making the load cell are quite simple, so that the load cell is economical, while still being permanently hermetically sealed for applications in harsh or extreme environmental conditions. The seal caps arranged on an external or outer surface are easily accessible for the original fabrication, and for later servicing if that should become necessary. The seal caps have only a minimal influence on the strain development in the sensing sections of the bending beam elements. Particularly, the stress and strain is substantially dissipated by elastic flexing of the sidewalls of the seal cap, so that the flat base or outer lid of the seal cap takes up essentially no stress. Thus, the strain development and strain measurement in the sensing sections of the bending beam elements is only minimally influenced, so that the load cell can be designed to handle a minimum load at or below 5 kg, or even down to 1 kg, with certifiable accuracy.